Wireless communication technology has evolved greatly over recent years. Recent studies have shown that the wireless spectrum in the U.S. is under-utilized, although much of the spectrum below 6 GHz has been assigned to licensees, or primary users (PUs). Therefore, spectrum sharing has been proposed to alleviate spectrum scarcity that prevents new wireless services from being deployed. Cognitive radio is a promising technology that can allows secondary users (SUs) to access the spectrum opportunistically on a non-licensed basis, provided that they do not harm the transmissions of PUs. To do that, cognitive radios operated by SUs may sense PU communications activity over the wireless channel in order to avoid a collision, which occurs if a radio device receives two simultaneous transmissions over the same frequency channel.
Cognitive radio technology enables opportunistic spectrum access (OSA). OSA is a new paradigm in wireless communications that allows secondary (unlicensed) users to opportunistically access temporarily idle licensed channels. These SUs generally need to follow a certain protocol that ensures that the secondary transmissions will not negatively impact any ongoing or future communications of the primary (licensed) users.
A commonly accepted scheme for protecting PUs from SU transmissions is called Listen-Before-Talk (LBT). In the LBT scheme, an SU device has to periodically interrupt its transmission (e.g., once every two seconds) and sense the channel for PU activity (transmissions). If the sensing outcome is positive (i.e., with high probability, the SU suspects that the channel is being used for PU transmission), the SU has to abort transmission over that channel and switch to another channel (in which case, the sensing process has to be conducted over the new channel). Otherwise, if the outcome is negative, the SU proceeds with its transmission until the next sensing attempt. Because of interference and noise, the outcome of the sensing process is not 100% accurate, so it may result in false positives (false alarm) and false negatives (mis-detection). False negatives are particularly significant, as they result in SU transmissions interfering with PU receptions.
In addition to their limitations arising from false alarms and misdetection, the LBT scheme inherently results in a reduction in the SU throughput because the SU has to interrupt its transmission to sense the spectrum for PU activities.